A photodiode has a pn-junction. The pn-junction is reversely biased in operation of the photodiode. Although various types of photodiodes have been fabricated, a typical photodiode has a bottom electrode deposited on a bottom surface of a substrate and a ring electrode deposited on a light-receiving region of the reverse side of the substrate. External light enters the light-receiving region and arrives at the pn-junction. The light excites plenty of pairs of electron and hole at the pn-junction. Since the pn-junction is reversely biased, electrons are attracted to the n-type region and holes are attracted to the p-type region. Thus, electric current flows in proportion to the intensity of the incident light.
FIG. 7 and FIG. 8 show a structure of a conventional PIN type photodiode having an InGaAs light-receiving layer. In the photodiode, an undoped InP buffer layer (2a), an undoped InGaAs light receiving layer (2b) and an undoped InP window layer (2c) have been epitaxially grown on an n.sup.+ -InP substrate (1) in succession. A p-type region (3) has been selectively formed in a central part of the diode by implanting p-type dopant ions or diffusing p-type dopant atoms on the undoped InP window layer. An annular p-side electrode (5) has been formed on the periphery of the p-type region (3).
The central part of the p-type region (3) enclosed by the annular p-side electrode (5) is a light receiving region. The peripheral part of the undoped InP window layer (2c) out of the annular p-side electrode (5) is coated with a protective film (8).
An n-side electrode (7) has been formed on the bottom side of the n.sup.+ -InP substrate (1). The pn-junction between the p-type region (3) and the undoped InGaAs light receiving layer (2b) is reversely biased by applying voltage between the p-side electrode (5) and the n-side electrode (7). Here, the reverse bias means that the n-side electrode (7) is connected to a positive electrode and the p-side electrode (5) is connected to a negative electrode of a DC current power source. Most of the voltage of the power source is applied to the pn-junction. A strong electric field is established at the pn-junction.
External light passes through an antireflection film (4) enclosed by the p-side electrode (5) and attains to the pn-junction in the undoped InGaAs light receiving layer (2b). Each photon of the light excites a pair of electron and hole. The electric field existing at the pn-junction pushes the electrons toward the n-side electrode (7), and the holes toward the p-side electrode (5). An electric current is induced by the flow of the electrons and holes. This current is often called a photocurrent.
Prior art has a drawback of which man is not still aware perhaps. In a conventional photodiode, external light also enters the peripheral part outside of the annular p-side electrode (5), because the protective film (8) is transparent to light. This peripheral part is now called a light non-receiving region or in brief, a non-receiving region, because it is undesirable that external light enters this region. Since the non-receiving region is only covered with a protective film (8) which is transparent to light, external light can pass through the protective film.
The light which has entered the peripheral, non-receiving region, also excites pairs of electron and hole. But there is no electric field at the non-receiving region, because the non-receiving region contains no pn-junction. The electrons and holes are not carried by an electric field. Thus, people may think that the electrons and holes would soon vanish by recombination without any bad effects. But this is wrong. The lifetime of the excited electrons and holes if pretty long. As the excited electrons and holes are not carried by an electric field, they are accumulated there. The carrier concentration becomes locally higher. Thus, the electrons and holes diffuse along with the gradient of the carrier (electrons and holes) concentration. Some of the diffused carriers attain to the pn-junction. Such electrons and holes also contribute to the photocurrent.
However, the movement by the diffusion is very slow, because of the low gradient of carrier concentration. Thus, it takes long time from the reception of light to the generation of the additional photocurrent. Such a slow drift of the carriers borne in the non-receiving region blunts the response of the photodiode. As shown in FIG. 6, the photocurrent of the photodiode is accompanied by a dull tail which is induced by the diffusion of carriers. In FIG. 6, the abscissa denotes the time in nanosecond (ns). The ordinate denotes the response of photocurrent of the photodiode.
The quick, forward, strong photocurrent is brought about by the light entering the central light receiving region enclosed by the annular p-side electrode (5). The dull, backward, weak tail of photocurrent is induced by the light entering the peripheral non-receiving region. The undesirable light delays the response of the photodiode.
In the case of the photodiode used for optical communication, the light emitted out of an optical fiber is converged by a lens system to a central receiving region of the photodiode. However, owing to the disorder of optical system, e.g. an axial misarrangement of lenses or optical fibers, part of the light emitted from the optical fiber sometimes deviates from the normal receiving region. The partial light enters the non-receiving region and generates extra pairs of electron and hole. The electrons and holes borne in the non-receiving region delays the response of the photodiode by adding the tail induced by the diffused carriers thereto.
Especially in the case of a photodiode for high speed response, the area of the light receiving region is usually very narrow in order to reduce the capacitance of the pn-junction. Thus, bigger proportion of the light enters the non-receiving region. The proportion of diffusion current in the photocurrent increases. The increment of the diffusion current deteriorates the response performance of the photodiode. These are problems of the photodiode mounted on a receiving side of an optical communication system. There are another problems also for a monitor photodiode mounted on a sending side. The photodiode of a sending side is sometimes installed to regulate the output power of a laser diode. The monitor photodiode detects the intensity of the light emitted from the rear end of the laser diode. The output of the monitor photodiode is compared with a standard value. Then, the driving current sent to the laser is reduced or increased to the direction of reducing the difference between the output and the standard value. By the negative feed-back, the intensity of the light emitted from the laser diode is stabilized to a definite value. In this case, if light from the laser diode enters also the non-receiving region, the partial light will induce an additional current owing to the diffusion of the carriers. The additional current delays the response of the monitor photodiode. The delay of the response retards the negative feed-back controlling. Thus, the output power of the laser diode will fluctuate to some extent.
The purpose of this invention is to provide a photodetector excelling in the response performance.
Another purpose of this invention is to provide a photodetector whose response includes no tail induced by the diffusion current.